1. Field of the Invention
The present invention relates to a printing apparatus that enables dots to be created in pixels arranged in a two-dimensional array and thereby prints an image on a printing medium. More specifically the invention pertains to a printing apparatus that significantly varies the maximum dimensions of the dot in two different directions that define the two-dimensional array.
2. Description of the Related Art
Ink jet printers, which create dots with several color inks ejected from a plurality of nozzles formed in a print head and thereby record an image, have been proposed as the output apparatus of the computer. The ink jet printers are widely used to print images processed by the computer in a multi-color, multi-tone expression. This printer generally allows expression of only two levels, that is, the dot on level and the dot off level, with regard to each pixel. The printer accordingly expresses the various tones of original image data by dispersion of dots.
One proposed technique reduces the weight of ink to create each dot, in order to ensure rich tone expression and high-quality printing in binary dot printers having only two levels with regard to each pixel. Creation of each dot by a reduced weight of ink enables an increase in number of dots possibly formed per unit area. This increases the number of tones expressible per unit area. The binary dot printer of this proposed technique, however, has the lowered printing speed, while enabling the rich tone expression.
Multilevel printers that enable tone expression of three or greater levels have been proposed as the technique of ensuring the rich tone expression as well as the improved printing speed. Examples of such multilevel printers include a printer that varies the quantity of ink or the density of ink to enable expression of three or more different densities with regard to each dot and a printer that enables a plurality of dots to be created in an overlapping manner in each pixel for multi-tone expression.
In the case of printing an image by a two-dimensional array of dots, the ideal shape of the dot is a circle to equalize the granularity of a resulting printed image in every direction. The quantity of ink ejected to form a substantially circular dot is rather fixed for each nozzle. In the case of formation of the substantially circular dots, there is accordingly a restriction in variation of the tone value expressible in each pixel by varying the quantity of ink ejected from the nozzle. In the binary dot printer using a single type of dot, there is a restriction in variation of the area of the dot formed according to the printing mode.